Prepare the quinoa - Prepare the quinoa according to the package directions. Spread out on a baking sheet, separating the grains, and set aside to cool and dry a bit. Be careful not to over-cook! You don't want the quinoa to be mushy.
Prepare the vegetables - Chop the parsley and mint leaves. Thinly slice the scallions. Core, peel, and deseed the tomato, then chop it in bite-sized pieces. Peel, deseed, and chop the cucumber.
Make the lemon dressing - Whisk together the olive oil, lemon juice, and salt.
Toss and Serve - Toss the quinoa and the vegetables with the lemon dressing. Pepper generously, and serve.
Notes
If you have the time, it's actually better to let this tabbouleh sit and become more infused with the lemony brightness of the dressing.